Ahypothetical bacterium swims among human intestinal contents until it finds a suitable location on the intestinal lining. it adheres to the intestinal lining using a feature that also protects it from bacteriophages and dehydration. fecal matter from a human in whose intestine this bacterium lives can spread the bacterium, even after being mixed with water and boiled. the bacterium is not susceptible to the penicillin family of antibiotics. it contains no plasmids and relatively little peptidoglycan. this bacterium derives nutrition by digesting human intestinal contents (in other words, food). humans lacking this bacterium have no measurable reproductive advantage or disadvantage relative to humans who harbor this bacterium. consequently, the bacterium can be properly described as which of the following?
(1). symbiont
(2). endotoxin
(3). mutualist
(4). commensal